Build a Secure Password Strength Checker in Python

A Python function that evaluates password strength based on length and character diversity, returning Weak, Moderate, or Strong.

Easy Python 3.9+ Jun 27, 2026 Strings & text 1 views 0 copies

password security regex validation

Python code

26 lines

Python 3.9+

import re

def password_strength(password: str) -> str:
    score = 0
    if len(password) >= 8:
        score += 1
    if re.search(r'[a-z]', password):
        score += 1
    if re.search(r'[A-Z]', password):
        score += 1
    if re.search(r'\d', password):
        score += 1
    if re.search(r'[!@#$%^&*(),.?":{}|<>]', password):
        score += 1

    if score <= 2:
        return "Weak"
    elif score == 3:
        return "Moderate"
    else:
        return "Strong"

if __name__ == "__main__":
    test_passwords = ["hello", "Password123!", "weak", "Str0ng!Pass"]
    for pwd in test_passwords:
        print(f"'{pwd}' -> {password_strength(pwd)}")

Output

stdout

'hello' -> Weak
'Password123!' -> Strong
'weak' -> Weak
'Str0ng!Pass' -> Strong

How it works

The function assigns one point for each of five criteria: length >=8, lowercase, uppercase, digit, and a special character. The score determines the strength rating. Using re.search with character classes makes checks concise and readable. This approach gives immediate feedback without external libraries.

Common mistakes

Forgetting to check for whitespace or empty strings, which would score 0 and return Weak.
Using a weak regex for special characters that misses common symbols like `@` or `#`.
Not handling Unicode or non-ASCII characters, which may cause false positives.

Variations

Use a single regex with lookaheads for a one-pass check.
Add a blacklist check against common weak passwords.

Real-world use cases

Enforcing minimum password strength during user registration in a web app.
Validating password reset tokens to prevent reuse of weak passwords.
Auditing existing stored hashes by rechecking strength of known plaintexts.

Build a Secure Password Strength Checker in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

Tutorials

Quizzes

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

Keep learning

Tutorials

Quizzes